1. Field of the Invention
The present invention relates to a radiation imaging apparatus and method for obtaining a radiation tomographic image of a breast by computer tomography (CT) and a radiation planar image appropriately displaying a condition of calcification in a breast.
2. Description of a Related Art
Conventionally, breast cancer examinations have been made by using palpation and ultrasonic diagnoses. However, in these examinations, only cancer cells that have been grown to some degree can be easily found. Accordingly, mammography for obtaining X-ray images, in which tumor masses and calcification produced in early breast cancer are relatively easily found, is used in combination. However, X-ray images obtained in mammography are simple planar images, and therefore, when the tumor mass part or the calcified part as a target of examination overlaps with tissues such as mammary gland, detection of a focus is difficult.
Accordingly, a radiation tomographic imaging apparatus for breast (mammography CT apparatus) using computer tomography (CT) that can obtain three-dimensional (3D) images has been developed. Even when a focus part overlaps with a radiopaque part, the mammography CT apparatus can make an accurate evaluation and has effect on early recognition of breast cancer.
As an example of a mammography CT apparatus, U.S. Pat. No. 6,480,565 B1 discloses a device for producing a three-dimensional tomographic mammography image of a breast of a patient. The device disclosed in U.S. Pat. No. 6,480,565 B1 comprises: a gantry frame; at least one motor for moving the gantry frame to form a data acquisition geometry; a source of radiation attached to the gantry frame to move with the gantry frame; a flat panel detector attached to the gantry frame to move with the gantry frame, the flat panel detector being disposed in a path of the radiation; and a support on which the patient rests while the mammography projection images are taken, the support supporting the patient such that the breast is disposed between the source of radiation and the flat panel detector; the support comprising a table on which the patient lies while the mammography projection images are taken; wherein the at least one motor moves the gantry frame so that the flat panel detector takes a volume scan of the breast; and wherein the table has two breast holes for both of the patient's breasts.
When breast cancer detection is performed by using the device disclosed in U.S. Pat. No. 6,480,565 B1, the patient rests face down on the table to allow two breasts to extend through the two breast holes, and one breast descends between the radiation source and the flat panel detector. The radiation source and the flat panel detector perform radiation imaging at each predetermined angular position while rotating together around a rotational axis, and thereby, the flat panel detector obtains radiation images of the breast in plural directions. The image signals obtained by the flat panel detector are transmitted to an image reconstruction and processing module. The image reconstruction and processing module three-dimensionally reconstructs the obtained radiation images to create a radiation tomographic image of the breast. Since the radiation tomographic image of the breast is a 3D image, a focus part at the rear side of the tissues such as mammary gland can be detected.
Since small tumor masses and calcification play an important part for early recognition in breast cancer examinations, it is important to sufficiently image them. In a radiation planar image obtained by the mammography apparatus, a calcified part exhibits image absorption equal to a bone, and a relatively small calcified part can be easily visualized. On the other hand, in a radiation tomographic image obtained by the mammography CT apparatus, a tumor mass can be detected, but detectability is not high for fine calcification because high-frequency information is deteriorated when the radiation tomographic image is created by reconstructing the plural radiation images.
Accordingly, it is conceivable that the tumor mass is detected in the radiation tomographic image and the fine calcified part is detected in individual radiation images. However, when obtaining plural radiation images to be used for creating the radiation tomographic image in the mammography CT apparatus, an extremely large number of times of imaging for obtaining three hundred images per one rotation, for example, are necessary. Therefore, in order to suppress the damage by the large number of times of exposure, radiation with low energy and low dose is selected to form a radiation cone beam to be used for production of radiation tomographic image. Accordingly, if radiation images obtained in the middle of formation of a radiation tomographic image in the mammography CT apparatus are singly used, fine calcification cannot sufficiently be detected in the individual images.